An optical mask is used to pattern a photoresist layer on a semiconductor substrate. The patterned photoresist layer on the semiconductor substrate forms a mask that is used in conjunction with wafer processing techniques to form devices and interconnect of an integrated circuit. The optical masking process that has dominated wafer processing for the past several decades comprises a quartz substrate patterned with a chrome layer. The optical mask is projection aligned to the semiconductor wafer and is optically reduced, typically in a 4:1 ratio. Radiation of a predetermined wavelength is projected through the mask. The predetermined wavelength is selected such that it passes through the quartz substrate readily. Chromed areas of the mask block the radiation from passing through the mask. The wavelength of radiation used directly relates to the critical dimensions that are achievable by the wafer process. In general, as the wavelength of radiation is reduced in the patterning process a corresponding reduction in the critical dimension can be realized. Thus, transistor and interconnect density have increased in part because of advances in photolithographic techniques in using smaller wavelengths of radiation for wafer patterning.
Radiation of a predetermined wavelength is projected to the optical mask. The radiation is projected to the surface of a semiconductor wafer such that the chromed areas of the mask prevent radiation from impinging on corresponding areas of the surface of the semiconductor wafer. As mentioned previously, the radiation typically impinges on a layer of photoresist that is placed on the surface of the semiconductor substrate. Both positive and negative photoresist have been used in wafer processing. In either case, the photoresist is sensitive to radiation such that areas that are exposed to the radiation are altered. A photoresist development process removes some of the photoresist leaving a patterned mask of photoresist that adheres to the surface of the wafer. Other process steps are then performed on the semiconductor wafer such as etching, doping, and deposition where the underlying areas on the wafer surface having the photoresist are masked or protected from these steps.
The semiconductor industry has had exceptional success in refining the simple chrome masking process to produce smaller geometry devices using smaller wavelengths of radiation but the process cannot be extended indefinitely. A paradigm shift occurs when radiation having wavelength less than approximately 157 nanometers is used in wafer processing. The problem is that radiation having a wavelength below 157 nanometers is readily absorbed by most materials including quartz. Furthermore, the resolution and depth of focus of an optical system become more of an issue at the smaller wavelengths when using a conventional lithographic approach. Ultimately it is the cost and yield of a manufacturing process that determines the suitability of a system for a particular critical dimension.
EUV (extreme ultraviolet) lithography is emerging as a viable solution for sub 0.05 micron critical dimensions. EUV lithography uses short wavelength (approx 13–15 nm) radiation that is reflected to a semiconductor wafer. The EUV mask and the other elements of the EUV lithographic system that transfer radiation to the semiconductor wafer are essentially high quality mirrors. Since EUV radiation is readily absorbed by most materials, the mask substrate should be specially designed with multiple layer pairs of molybdenum and silicon that is highly reflective at the target extreme ultraviolet wavelength. The circuit features patterned on the reflective substrate should be absorbing at EUV wavelength to form an image of the features with high contrast. This is analogous to the chrome masking in an optical mask. Also, diffraction effects are a problem in producing sharp delineation between areas on a semiconductor wafer surface receiving and not receiving EUV light.
Accordingly, it is desirable to provide an extreme ultraviolet lithographic mask that is both manufacturable and capable of producing sub 0.05-micron critical dimensions. In addition, it is desirable to minimize second order effects such as shadowing and sidewall reflection on the EUV mask. Furthermore, other desirable features and characteristics of the present invention will become apparent from the subsequent detailed description and the appended claims, taken in conjunction with the accompanying drawings and the foregoing technical field and background.